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LARGE SCALE MAGNETIC FIELD MEASUREMENTS AND MAPPING
K. Henrichsen
To cite this version:
K. Henrichsen. LARGE SCALE MAGNETIC FIELD MEASUREMENTS AND MAPPING. Journal de Physique Colloques, 1984, 45 (C1), pp.C1-937-C1-942. �10.1051/jphyscol:19841191�. �jpa-00223668�
JOURNAL DE PHYSIQUE
Colloque C1, supplement a u n o 1, Tome 45, janvier 1984 page C1-937
LARGE SCALE MAGNETIC F I E L D MEASUREMENTS AND MAPPING
K . N . Henrichsen
LEP Division, C . E.R. N., 121 1 Geneva 23, Switzer Zand
R6sum6 - Cet a r t i c l e f a i t l e p o i n t sur l e s techniques l e s p l u s r6centes en m a t i s r e de mesures magngtiques 2 grande 6 c h e l l e, en i n d i q u a n t 1 es p r i n c i p a u x problgmes l i 6 s 2 l a c a r t o g r a p h i e du champ magn6tique:-Le d6veloppement de l a m i c r o 6 l e c t r o n i q u e au cours des dernigres ann6es a Cree une r 6 v o l u t i o n dans l e s
systgmes de mesures magngtiques. L16mergence de m a d r i e l s e t 1 o g i c i e l s modu- l a i r e s , a i n s i que 1 'usage rgpandu de standards i n d u s t r i e l s o n t i n f l u e n c 6 1 'a r - c h i t e c t u r e des syst&nes de mesure. L 1 i n t 6 g r a t i o n de microprocesseurs dans 1 ' 6 - quipement e t l a mise en oeuvre d'algorithmes modernes o n t am6lior6 l e s p e r f o r - mances des sondes de mesure e t s i m p l i f i 6 l e u r c a l i b r a t i o n . Tout systgme mo- derne pr6sente Ggalement l a p o s s i b i l i t 6 d'analyse e t de v i s u a l i s a t i o n en l i g n e des r e s u l t a t s de mesure, a i n s i que l a d 6 t e c t i o n e t l e d i a g n o s t i c pr6coces des pannes.
A b s t r a c t - The paper i l l u s t r a t e s the most r e c e n t techniques i n t h e f i e l d o f l a r g e - s c a l e magnetic measurements. The main problems r e l a t e d t o f i e l d mapping
a r e discussed. The development o f m i c r o e l e c t r o n i c s d u r i n g t h e l a s t few years has c r e a t e d a r e v o l u t i o n i n magnetic measurement systems. Modular hardware and software as w e l l as w i d e l y used i n d u s t r i a l standards have i n f l u e n c e d t h e a r - c h i t e c t u r e o f measurement systems. The use o f embedded microprocessors i n t h e equipment and t h e a p p l i c a t i o n o f modern algorithms have improved t h e perform- ance o f measurement probes and s i m p l i f i e d t h e i r c a l i b r a t i o n . A modern system a l s o provides f a c i l i t i e s f o r o n - l i n e a n a l y s i s and d i s p l a y o f measurement re- s u l t s as w e l l as e a r l y d e t e c t i o n and i n d i c a t i o n o f equipment f a u l t s .
I - INTRODUCTION
The problem i n 1 arge-scal e magnetic measurements i s t o o b t a i n t h e necessary accuracy f o r the f i e l d map w i t h i n t h e p e r i o d a l l o c a t e d t o t h e measurements. The time i s usual- l y l i m i t e d when the measurements a r e scheduled sometfme near t h e end o f a construc- t i o n p e r i o d and even more, i f t h e magnet has already been i n s t a l l e d as p a r t o f an e x i s t i n g a c c e l e r a t o r and t h e measurements must be performed d u r i n g periods of access t o t h e machine. I n a d d i t i o n , i t i s important t h a t t h e measurements are f u l l y complet- ed and c e r t i f i e d c o r r e c t b e f o r e t h e i n s t a l l a t i o n o f detectors, vacuum chambers and o t h e r components w i t h i n t h e magnetic f i e l d volume.
These requirements c a l l f o r a d e t a i l e d a n a l y s i s o f t h e task, a c a r e f u l design o f t h e measurement equipment and a c l e a r idea o f the way t o present and judge the measure- ment r e s u l t s . Some model work i s o f t e n u s e f u l and may save considerable time.
It i s c u r i o u s t o note t h a t w h i l e t h e measurement methods have remained v i r t u a l l y un- changed f o r a very l o n g period, t h e equipment has been s u b j e c t t o continued develop- ment. I n t h e f o l l o w i n g , o n l y the two most commonly used methods w i l l be mentioned.
F o r a more complete d e s c r i p t i o n o f t h e many o t h e r e x i s t i n g measuring methods, reference i s made t o two c l a s s i c a l b i b l i o g r a p h i c a l reviews [1,21.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19841191
(21-938 JOURNAL DE PHYSIQUE
I 1 - THE FLUXMETER METHOD
Based on t h e i n d u c t i o n law, t h i s method i s t h e most a n c i e n t 131 o f t h e c u r r e n t l y used methods f o r magnetic measurements, b u t i t can be very p r e c i s e 141. It i s a l s o t h e most p r e c i s e method f o r t h e measurement o f t h e d i r e c t i o n o f magnetic f l u x l i n e s . Measurements are performed e i t h e r u s i n g f i x e d c o i l s i n a dynamic magnetic f i e l d o r by moving t h e c o i l s i n a s t a t i c f i e l d . The c o i l geometry i s o f t e n chosen t o s u i t a par- t i c u l a r measurement. One example i s t h e F l u x b a l l [51 which was o f r a t h e r complex c o n s t r u c t i o n and which was used f o r p o i n t measurements i n inhomogeneous f i e l d s . The c o i l method i s p a r t i c u l a r l y s u i t e d f o r measurements w i t h l o n g c o i l s i n beam g u i d i n g magnets [61, where the p r e c i s e measurement o f t h e f i e l d i n t e g r a l along t h e p a r t i c l e t r a j e c t o r y i s the main problem. I n t h i s case, t h e geometries o f t h e measurement c o i l s a r e chosen so as t o 1 in k w i t h s e l e c t e d f i e l d components f71.
I n d u c t i o n c o i l s were o r i g i n a l l y used w i t h a b a l l i s t i c galvanometer and l a t e r on w i t h more e l a b o r a t e fluxmeters [8, 91. The c o i l method was improved considerably w i t h t h e i n t r o d u c t i o n o f t h e c l a s s i c a l e l e c t r o n i c i n t e g r a t o r , t h e M i l l e r i n t e g r a t o r , b u t i t remained necessary t o employ d i f f e r e n c e methods i n p r e c i s i o n measurements C101. The advent o f d i g i t a l v o l t m e t e r s made f a s t absolute measurements p o s s i b l e and t h e M i l l e r i n t e g r a t o r has remained t h e most popular fluxmeter. With t h e development o f s o l i d s t a t e d.c. a m p l i f i e r s , t h i s i n t e g r a t o r has become inexpensive and i s o f t e n used i n m u l t i - c o i l systems 111, 12, 131. The main problems w i t h t h e M i l l e r i n t e g r a t o r are now r e l a t e d t o t h e i n t e g r a t i n g capac4tor L141.
A few e l e c t r o n i c i n t e g r a t o r s have been developed by i n d u s t r y and a r e commercially a v a i l a b l e . They are, however, r e l a t i v e l y expensive and t h e choice i s r a t h e r l i m i t e d . I n r e c e n t years, a new t y p e o f e l e c t r o n i c i n t e g r a t o r has been developed, which i s based on a h i g h q u a l i t y d.c. a m p l i f i e r connected t o a voltage-to-frequency convert- e r and a counter. This system i s w e l l adapted t o d i g i t a l c o n t r o l b u t imposes l i m i t s on t h e maximum s i z e o f t h e i n p u t s i g n a l , o r i n o t h e r words t h e r a t e o f change of t h e f l u x .
The s e n s i t i v i t y o f t h e fluxmeter method depends on t h e e f f e c t i v e c o i l surface and the q u a l i t y o f t h e i n t e g r a t o r . The c o i l - i n t e g r a t o r assembly can be c a l i b r a t e d t o an ac- curacy o f 10 ppm i n a homogeneous magnetic f i e l d w i t h reference t o a nuclear magnetic resonance probe.
I 1 1 - THE HALL PROBE METHOD
Nowadays, t h e most commonly used method i n f i e l d mapping 115, 16, 17, 181 i s t h e H a l l probe method. It i s t h e simpler and f a s t e r o f t h e two methods considered here, b u t a l s o t h e l e s s precise.
E.F. H a l l made h i s discovery i n 1879 1191 and i n 1910, magnetic measurements were performed u s i n g t h i s method 1201. It was, however, o n l y around 1953 t h a t s u i t a b l e semiconductor m a t e r i a l s were developed C21, 221 and since then the method has been used e x t e n s i v e l y .
Several f a c t o r s s e t l i m i t s on t h e o b t a i n a b l e accuracy, t h e most serious being t h e temperature c o e f f i c i e n t o f the H a l l voltage. Temperature s t a b i l i z a t i o n i s u s u a l l y em- ployed i n o r d e r t o overcome t h i s problem 118, 23, 241. It may a l s o be taken i n t o ac- count i n the probe c a l i b r a t i o n by m o n i t o r i n g the temperature d u r i n g measure-
ments [251. The temperature c o e f f i c i e n t depends, however, on t h e l e v e l o f t h e magnet- i c f i e l d 1251, so r e l a t i v e l y complex c a l i b r a t i o n t a b l e s a r e needed. Another complica- t i o n can be t h a t o f t h e p l a n a r H a l l e f f e c t 1261, which makes t h e measurement o f a weak f i e l d component normal t o the plane o f t h e Hal 1 p l a t e problematic i f a s t r o n g f i e l d component i s present p a r a l l e l t o t h i s plane. Many p o s s i b l e remedies have been proposed 1271 b u t they a r e a l l r e l a t i v e l y d i f f i c u l t t o apply. L a s t b u t n o t l e a s t i s t h e problem o f t h e r e p r e s e n t a t i o n o f t h e c a l i b r a t i o n curve since t h e H a l l c o e f f i c i e n t v a r i e s w i t h t h e magnetic f i e l d . The H a l l probe o f t h e c r u c i f o r m type 1281 shows a b e t t e r l i n e a r i t y and has a smaller a c t i v e surface than the usual r e c t a n g u l a r plate.
I t s magnetic c e n t r e i s , therefore, b e t t e r defined, so i t i s p a r t i c u l a r l y w e l l s u i t e d
Table 1 - H a l l probe c h a r a c t e r i s t i c s Type:
Siemens
Nominal H a l l v o l t a g e a t 1 T Temperature Geometry I Current and nominal c u r r e n t c o e f f i c i e n t 1mA1 / ImV1 Ippm/"C1 B FC32
SBV 579 SBV 601 SBV 601-S1 SBV 585 SBV 585-S1
1 !! 1 !i 1
::1;1
;.61
c r u c i f o r m
c r u c i f o r m -500 1.0
c r u c i f o r m 200 - 30 1.0
c r u c i f o r m 100 -900 0.02
I c r u c i f o r m 100 50 - 70 0.02
f o r measurements i n s t r o n g l y inhomogeneous f i e l d s . Special types, which have smaller temperature dependence, a r e a v a i l a b l e on t h e market, b u t these probes show a lower s e n s i t i v i t y . Table 1 g i v e s some t y p i c a l c h a r a c t e r i s t i c s o f v a r i o u s H a l l probes 1291.
The measurement o f t h e H a l l v o l t a g e s e t s a 1 i m i t o f about 1 G on t h e s e n s i t i v i t y and r e s o l u t i o n o f t h e measurement i f conventional d i r e c t c u r r e n t e x c i t a t i o n i s appl i e d t o t h e probe. The s e n s i t i v i t y can be improved considerably i f a l t e r n a t i v e c u r r e n t e x c i - t a t i o n i s used C30, 311. A good accuracy a t low f i e l d s can then be achieved by em- p l o y i n g synchronous d e t e c t i o n techniques f o r t h e measurement o f t h e H a l l voltage.
H a l l p l a t e s are u s u a l l y c a l i b r a t e d i n a magnet i n which t h e f i e l d i s measured simul- taneously u s i n g a nuclear magnetic resonance probe. The c a l i b r a t i o n curve i s most commonly represented i n t h e form o f a polynomial o f r e l a t i v e l y h i g h order (9 o r more) f i t t e d t o a s u f f i c i e n t l y l a r g e number o f c a l i b r a t i o n points. This r e p r e s e n t a t i o n has t h e advantage o f a simple computation o f t h e magnetic i n d u c t i o n from a r e l a t i v e l y small t a b l e o f c o e f f i c i e n t s .
A p h y s i c a l l y b e t t e r r e p r e s e n t a t i o n i s t h e use o f a piecewise cubic i n t e r p o l a t i o n through a s u f f i c i e n t number o f c a l i b r a t i o n p o i n t s which were measured w i t h h i g h pre- c i s i o n . This can be done i n t h e form o f a simple Lagrange i n t e r p o l a t i o n o r even bet- t e r w i t h a cubic s p l i n e function. The advantage o f t h e s p l i n e f u n c t i o n comes from i t s minimum c u r v a t u r e and i t s " b e s t approximating" p r o p e r t i e s C321. The f u n c t i o n a d j u s t s i t s e l f e a s i l y t o non-analytic f u n c t i o n s and i s very w e l l s u i t e d t o t h e i n t e r p o l a t i o n from t a b l e s o f experimental data. The cubic s p l i n e f u n c t i o n i s a piecewise polynomial o f t h i r d degree passing through t h e c a l i b r a t i o n p o i n t s and d e f i n e d such t h a t t h e de- r i v a t i v e o f t h e f u n c t i o n i s continuous a t these p o i n t s . Very e f f i c i e n t a l g o r i t h m s can be found i n t h e 1 it e r a t u r e 1331. The c a l c u l a t i o n o f t h e polynomial c o e f f i c i e n t s may be somewhat time-consuming b u t need o n l y be done once a t c a l i b r a t i o n time. The coef- f i c i e n t s ( t y p i c a l l y about 60 f o r t h e b i p o l a r c a l i b r a t i o n o f a c r u c i f o r m H a l l p l a t e ) can be e a s i l y s t o r e d i n a modern device [241 and the subsequent f i e l d c a l c u l a t i o n s a r e very f a s t . The q u a l i t y o f t h e c a l i b r a t i o n f u n c t i o n can be v e r i f i e d from f i e l d va- l u e s measured between the c a l i b r a t i o n p o i n t s . A w e l l designed Hall-probe assembly can be c a l i b r a t e d t o an accuracy o f 100 ppm.
I V - MEASUREMENT BENCHES
The mechanical gear f o r moving and p o s i t i o n i n g t h e measurement probes i n s i d e t h e mag- n e t i c volume i s u s u a l l y designed f o r each s p e c i f i c a p p l i c a t i o n , t h e shape and symme- t r i e s o f t h e magnetic f i e l d determining t h e probe movements and t h e choice o f coor- d i n a t e system. A few " u n i v e r s a l " f i e l d mappers have been b u i l t [23, 34, 351. I n ge- n e r a l , s t a n d a r d i z a t i o n i s d i f f i c u l t t o achieve b u t may occur i n t h e design o f modular probe assemblies and rail-arrangements f o r c a r r i a g e movements 1361. This s i t u a t i o n may change when i n d u s t r i a l r o b o t s w i l l become p a r t o f our measurement systems.
An example o f a r e c e n t l a r g e - s c a l e f i e l d mapping was t h e measurement C37, 381 o f t h e CERN-UA1 d i p o l e magnet w i t h i t s u s e f u l i n n e r f i e l d volume o f about 70 m3. F i g u r e 1 shows t h e measuring gear mounted i n s i d e t h e dipole. A l a r g e aluminium frame c a r r i e d a
C1-940 JOURNAL DE PHYSIQUE
t o t a l o f 70 H a l l probes, 66 o f which measured the main component o f t h e f i e l d a t 10 cm i n t e r v a l s around t h e p e r i p h e r y o f t h e frame, w h i l e t h e remaining f o u r measured t h e two perpendicular components f o r checking purposes. The frame t r a v e r s e d the f i e l d volume a t 10 cm i n t e r v a l s and f i n a l l y moved v e r t i c a l l y a t e i t h e r end o f t h e
volume so t h a t t h e o u t s i d e surface o f ha1 f o f t h e complete f i e l d volume was measured. Assuming symmetry w i t h t h e o t h e r h a l f o f t h e volume t h e t o - t a l f i e l d i n the magnet was evaluat- ed from these boundary condi- t i o n s [391. This type o f f i e l d eva- l u a t i o n can be done w i t h reasonable accuracy i f t h e f i e l d shape i s n o t too complex.
A s p e c i a l probe a r r a y was necessary f o r more p r e c i s e measurements along t h e beam p a t h over a t o t a l l e n g t h o f 12 m. 75 H a l l p l a t e s measured the f i e l d i n a l l t h r e e dimensions on a mesh o f 50 mm.
Another example was t h e measure- ment [401 o f t h e l a r g e angle magnet- i c spectrometer (LAMS) which forms p a r t o f t h e CERN-UA2 experiment.
T h i s magnet has a smaller volume b u t a more complex f i e l d pattern, which favoured t h e choice o f a s ~ h e r i c a l
coordinate system. The meaiurement Fig. 1 - UA1 f i e l d mapper arrangement i s shown i n Fig. 2. A
3 m l o n g r a i l was f i x e d a t one end, p i v o t i n g about t h e magnet vertex. I t c a r r i e d a H a l l - p l a t e assembly f o r measurements i n t h r e e dimensions, t r a v e l l i n g along the r a i l . I n a d d i t i o n t o t h e H a l l - p l a t e measurements, a 2.75 m l o n g i n t e g r a t i n g i n d u c t i o n c o i l was employed i n o r d e r t o monitor the measurement q u a l i t y . Due t o t h e very inhomogene- ous f i e l d , i t was pecessary t o take i n t o account t h e v a r i a t i o n i n w i d t h along t h e l e n g t h of t h i s c o i l . The same equipment, b u t w i t h a d i f f e r e n t support s t r u c t u r e , was used f o r t h e f i e l d measurements o f t h e t o r o i d a l magnets mounted a t each end o f t h e spectrometer.
Fig. 2 - UA2 f i e l d mapper
V - SYSTEM ARCHITECTURE
The s t r u c t u r e o f measurement systems has grown i n complexity as t h e measurement tasks increased i n size. The need f o r automation c a l l e d f o r t h e use o f d i g i t a l e l e c t r o n i c techniques. As i n many o t h e r f i e l d s , t h e advent o f t h e mini-computer was t h e f i r s t step i n t h e d i r e c t i o n o f f u l l automation. The hard-wired l o g i c c i r c u i t s designed and b u i l t f o r a s p e c i f i c task were replaced by programmable systems which were commer- c i a l l y a v a i l a b l e . By connecting h i s measurement equipment t o t h e computer, t h e user c o u l d p r o f i t from the f l e x i b i l i t y o f a programmable system and from t h e advantages o f mass data storage i n a p o r t a b l e form, s u i t a b l e f o r l a t e r processing on a l a r g e com- puter. On-1 i n e data hand1 i n g combined w i t h simultaneous graphic d i s p l a y o f measure- ment r e s u l t s provided a powerful t o o l f o r e a r l y d e t e c t i o n o f f a u l t s i n t h e measure- ment equipment o r i n t h e o p e r a t i o n o f it. An i m p o r t a n t step forward i n standardiza- t i o n was t h e use o f CAMAC equipment l i n k i n g t h e measurement machines t o t h e compu- t e r Ell, 13, 16, 18, 351.
The microprocessor r e v o l u t i o n has now i n t u r n i n f l u e n c e d t h e design o f magnetic meas- urement equipment. Embedded microprocessors discharge t h e c o n t r o l computer t o a l a r g e extent. Tasks which consume processing time and memory space can be delegated t o microprocessors d i s t r i b u t e d w i t h i n t h e system. This a l l o w s a modular s t r u c t u r e o f t h e equipment which makes t h e design and programming o f t h e measurement system easy and provides an e f f i c i e n t use o f resources. The necessary data-bases a r e 1 ocated w i t h i n t h e equipment modules b u t a r e accessible from t h e c o n t r o l l e r f o r checking and i d e n t i - f y i n g purposes. Data r e d u c t i o n can a l s o be done l o c a l l y and may 1 i m i t considerably t h e amount o f data transmission w i t h i n t h e system. Diagnostic programs may be r e s i - d e n t i n t h e modules and w i l l a l l o w t h e s p e c i a l i s t t o perform t h e running-in and de- bugging o f h i s equipment through t h e c o n t r o l l e r . This form o f access makes mainten- ance and r e p a i r e a s i e r too. The use o f d i g i t a l r a t h e r than analog m u l t i p l e x i n g tech- niques solves many problems r e l a t e d t o e a r t h connections, c o n t a c t p o t e n t i a l s and com- mon mode signals.
The a p p l i c a t i o n o f w i d e l y used
i n d u s t r i a l standards, such as MEASUREMENT SYSTEM ARCHITECTURE
t h e General Purpose I n t e r f a c e Bus (GPIB), IEEE standard
NO. 488-1978, o r t h e computer HARDWARE :
I SOFTWARE :
communication standard RS-232-C f o r t h e i n t e r c o n n e c t i o n o f t h e v a r i o u s equipment modul es, w i l l
K E y B o A H o
I
l a r g e l y f a c i l i t a t e t h e use o f
commercially a v a i l a b l e i n s t r u - BASIC
ments and w i l l a l l ow an extreme ::KN:I", CONTROLLER PASCAL f l e x i b i l i t y even i n a complex
system. An example o f a micro-
ASCII - CHARACTER STRINGS
processor-based equipment as- COMPUTER - LINK
sembly i s shown i n Fig. 3. - - - -
A d u a l - l e v e l s t r u c t u r e was cho- sen. The work s t a t i o n a t opera- t o r l e v e l communicates w i t h t h e v a r i o u s microprocessors a t
AsstMsLIEs
EQUIPMENT ( EQT 1 EPT / EB / u PROCESSOR R1 ELjj :::::
equipment l e v e l throuqh t h e
G P I B ' W ~ ~ C ~ forms t h e Lackbone L 1 I l P
o f t h e system. Messages con- s i s t i n g o f s t r i n g s o f p r i n t a b l e
characters a r e t r a n s m i t t e d b i - Fig. 3 - Microprocessor-based
d i r e c t i o n a l l y through t h e measurement equipment
GPIB. D i r e c t communication
between modules a t equipment l e v e l as w e l l as simultaneous transmission t o two o r more modules i s a l s o possible. The o n l y disadvantage o f t h e GPIB standard i s i t s d i s t a n c e r e s t r i c t i o n o f about 20 m.
Cl-942 JOURNAL DE PHYSIQUE
The GPIB-controller can, depending on the needs f o r f l e x i b i l i t y o r speed, be pro- grammed e i t h e r u s i n g an i n t e r p r e t i v e language l i k e BASIC o r a compiled language l i k e PASCAL. A 1 arge v a r i e t y o f GPIB-controllers, ranging from t h e simple desk c a l c u l a t o r t o t h e powerful m u l t i - t a s k i n g minicomputer, are a t p r e s e n t a v a i l a b l e on t h e market. A number o f systems such as t h e one described a r e p r e s e n t l y used f o r t h e various t e s t s and measurements o f t h e LEP magnets C411.
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